This invention relates to an interactive enquiry system having a distributed data base and which may be used, for example, in a seat reservation and/or ticketing system.
Computerized reservation and ticketing systems in the past have comprised a central processor used to control a central data base. A number of remote terminals, normally consisting of a keyboard and display or printer, were connected to the central processor. Whenever the ticketing/reservation clerk wished to make an enquiry or conduct a transaction it was necessary for a connection to be established with the central data base.
Such an arrangement suffers from three disadvantages. Firstly, the cost of establishing the connection between the remote terminal and the host computer is not inconsiderable and is increasing all the time. Secondly, the time required to establish the connection adds to the response time of the clerk to answer a query or complete a transaction. Thirdly, the whole system is dependent upon the reliability of the communication links.
The Complete Specification of our co-pending Application for Letters Patent No. 16749/74, now UK Pat. No. 1437883 describes a ticketing system in which a number of local data bases are established so that transactions can be completed without the need for accessing the central data base for every transaction. Such an approach clearly requires local storage for the local data base and a local processing capability for the control of that local data base and the terminals connected to it. Therefore when considering the relative costs between a centralized data base system and a distributed data base system, some balance must be struck between the costs of local storage and the costs of communication. All other things being equal, faster response times represent reduced costs due to increased productivity on the part of the ticketing/reservation clerk.
The present invention is concerned with a distributed data base system employing storage management arrangements which tend to reduce the need for excessively large amounts of local storage. Methods of local storage management based upon last use or recency of use are known in computer systems employing a cache memory. Last use or recent use are however, not the best criteria for choosing data items to be stored in a local data of an interactive enquiry system.